Lithium ion secondary batteries have been widely used as power sources for portable equipment such as VTR cameras, notebook computers, and mobile phones, and are now being used in wide fields of game machines, electric power tools, power-assisted bicycles, etc. Further, lithium ion secondary batteries are gathering attention as power sources for electric vehicles due to their high energy densities.
In order to deal with environmental issues, the automobile industry are officially developing zero-emission electric vehicles using only batteries as power sources, and HEVs (Hybrid Electric Vehicles) and PHEV (plug-in Hybrid Electric Vehicles) using internal-combustion engines and batteries as power sources, which have partially reached practical use. Moreover, the automobile industry are also developing power source batteries.
The HEVs and PHEVs, which assist the acceleration forces of vehicles by motor driving, need to have high levels of assisting force, and high outputs of power source batteries are thus required for the HEVs and PHEVs. Further, increasing the energy density of a battery makes it possible to reduce the size of the battery and to increase the driving distance of a PHEV by one charge.
When lithium ion secondary batteries are used as, for example, power sources for HEVs or PHEVs, high output batteries are necessary since momentary charging and discharging are often performed repeatedly at current values of not smaller than approximately 10 C, depending on a method of controlling the system, when vehicles are driving. The charging and discharging reactions of a lithium ion secondary battery are performed in the vicinity of the interface between active material particles and an electrolyte in an electrode. Increasing the output of the lithium ion secondary battery requires a sufficient reaction area of the active material particles and the electrolyte and a sufficient amount of electrolyte near the active material particles to be provided in the reaction region of the active material particles and the electrolyte.
Further, increasing the energy density of a lithium ion secondary battery requires increasing the amount of active material particles to be filled into an electrode for use in the lithium ion secondary battery. The electrode includes mixed particles of the active material particles, conductive assisting agent particles, and a binder.
As methods for increasing the amount of active material particles to be filled into the electrode, there are a method (see PTL 1) in which voids among the mixed particles are reduced as much as possible to increase the particle filling rate by controlling the diameters and the shapes of the active material particles, and a method in which active material particles are filled into a predetermined area as much as possible by increasing the thickness of an electrode.